Push-pull audio amplifier circuit



Oct. 8, 1940. D, FOSTER 2,217,269 7 PUSH-PULL AUDIO AMPLIFIER CIRCUIT Filed Nov. 24, 1957 INVENTOR 01/015) F- FOSTER BY WQMM ATTO R N EY Patented Oct. 8, 1940 UNITED STATES PUfiH-PULL AUDIO AMPLIFIER CIRCUIT Dudley E. Foster, South Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 24, 1937, Serial No. 176,188

ll Claiml My present invention relates to audio amplifiers of the push-pull type, and more particularly to a push-pull audio amplifier of the class AB type.

One of the main objects of my invention is to provide a push-pull audio amplifier of the class AB type, and which amplifier employs a low impedance coupling element between an audio frequency driver tube and the amplifier input electrodes; the audio amplifier stage being characterized by its inclusion of independent control grid bias resistors.

Another important object of my invention is to provide in a push-pull audio amplifier of the class AB type, a self-biasing network comprising a pair of resistors individual to each of the space current paths of the push-pull connected tubes whereby minimum distortion is obtained without exceeding the safe dissipation of the tubes.

Still other objects of the invention are to improve generally the efficiency of push-pull audio networks of the class AB type, and more especially to provide such amplifier networks in a reliable manner, and in a form so that the amplifier networks economically constructed and assembled for use in audio systems.

The novel features which I believe to be characteristic oimy invention are set forth in particularity in the appended claim; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into efiect.

In the drawing:

Fig. l graphically shows the E -I characteristic of a class AB network embodying my invention, and

Fig. 2 shows a circuit diagram of a push-pull class AB network employing the invention.

Reference is first made to Fig. l which shows the Eg-Ip characteristic of an audio amplifier tube; it is to be understood that the full line Y shows a typical characteristic for an electron discharge tube used as an audio amplifier. The vertical dotted line denotes the zero control grid bias; the point A denotes the point on the characteristic Y at which an audio amplifier of the class type will be operated. Point 13 shOWS the control grid bias which is required for class B operation of an audio amplifier, and the bias which is to be employed is sufficiently negative to provide plate current cut-o-fi in the absence of impressed audio current. An audio amplifier of the class AB type is one which employs a normal negative bias on the amplifier control grid such that in the absence of impressed audio waves the amplifier will be biased to a point AB on the characteristic Y. 5 The vertical dotted line passing through point AB represents the reference line with respect to which the audio wave amplitude is considered. It will be seen that the effect of the audio waves W is to draw grid current on the positive swings, and increase the control grid bias on the audio amplifier grids so that they approach plate current cut-off. Of course, the situation illustrated in Fig. l is true of audio amplifier tubes arranged in push-pull relation, the amplifier tubes merely 15. being considered in reciprocal relation in that case.

Ln Fig. there is shown a push-pull audio amplifier network of the class AB type. Let it be assumed that the network is employed between any source of audio frequency currents and a reproducer. For example, the network could be employed between the reproducer of a radio receiver and the demodulator output circuit. Of course, it could be employed between the microphone of a public address system and the loudspeaker thereof; or, it may be utilized between the electrical pick-upof a phonograph and the loudspeaker oi the latter.

A push-pull audio amplifier of the class AB type will employ an audio frequency driver tube between the source of audio frequency currents and the input transformer of the push-pull stage. The audio driver tube may be any desired type of audio frequency amplifier circuit capable of delivering audio power, so that the signal control grids of the push-pull tubes can be suitably driven through a low impedance transformer.

It is not believed necessary to show such a driver tube, since those skilled in the art are fully acquainted with the construction of such a stage. The input transformer i of the push-pull stage is to be understood as having a relatively low impedance. The transformer primary winding 2 is connected between the output electrodes of the audio frequency driver tube, and the transformer secondary winding has its mid-point at ground potential. The control grids 4 and 5 of audio amplifier tubes 4 and 5 respectively are connected to opposite ends of the secondary 5 winding 3.

The output transformer 6 has its primary winding 5 arranged so that its opposite terminals are connected to the plates 8 and 9 of the tubes 4 and 5 respectively. The mid-point of primary winding 1 is connected to a point of proper positive potential of a direct current source. The latter is not shown, but it is to be understood that it may be any desired direct current source; for example, it may be the power supply bleeder resistor of the customary power supply circuit of a radio receiver. The secondary winding H! of the output transformer 6 may be connected to any desired type of reproducer.

The cathode ll of tube is connected to ground through a path including resistor 28 and resistor 2| arranged in series. The direct current voltage drop across resistors. 2|] and 2| provides the negative bias for the control grid 5 of tube 5'; the audio frequency by-pass condenser 30 is connected between thev cathode end of resistor 2|) and ground. The path including resistors All and 2|, arranged in series between the cathode 50 of tube 4 and ground, provides the negative bias for the control grid 4. An audio frequency by-pass condenser 3| is connected between the cathode end of resistor 46 and ground. The direct current voltage drop across resistors 48 and 2| provides the negative bias for'the control grid 4 of tube 4.

Before explaining the advantage of utilizing the individual self-biased resistors 28 and ll: in the cathode leads of the tubes i and 5, let it be assumed that the push-pull tubes utilize a common self-bias resistor. The minimum value of such a common self-bias resistor would be governed by the permissible plate dissipation of the tubes used. The permissible plate dissipation of a tube is the maximum power which can be dissipated from the plate electrode, chiefly by thermal radiation, without the plate attaining a temperature such that continued operation is harmful to the tube. The plate dissipation is equal to the difference between the input pcwer to the plate, and the useful power delivered to the load. The input power is in turn the product of the direct current drawn by the plate and the direct current voltage applied to the plate. -As the bias is reduced, the useful output and the plate current rise, but a point is soon reached where the power which must be dissipated by the plate is too great for continued safe opera- .tion of the tube. However, less distortion results if the magnitude of the resistor is decreased below the value set by permissible plate dissipation. The distortion arises because during the time that tube 5' would be drawing grid current, the signal voltage on tube 4 is most highly negative. This follows from the fact that the audio waves are applied to the control grids of the push-pull tubes in phase opposition relation. It is pointed out that the tube 5 would be drawing current during the portion of the cycle of maximum excitation in a positive sense on the control grid 5. When the tube 5 thus draws grid current, it follows that there would be a high current flow through the common self-bias resistorjthus, there would be a high negative bias positive, the control grid 4 of tube 4' is negative thereby decreasing the bias of that tube. Resistor 2|, arranged in series with each of resistors 2|! and 40, is added so that the change in bias is decreased during the audio signal cycle. When the grid 5 is swung to its maximum positive potential there will be a large drop of potential across 20--2| due to grid current flow. Hence, grid 4 will be biased highly negative thereby reducing the plate current of tube 4' toa low value. The decreased current flow through 482| produces a decrease of the negative bias on grid 4. This explains how the present biasing circuit functions to reduce the bias on the grid of each tube as the grid of the opposite tube is swung highly positive. In a push-pull'amplifier of the prior art a single bias resistor common to both tubes need not be bypassed to prevent degeneration of the fundamental applied frequency. Witha separate bias resistor in each cathode, degeneration will take place unless the resistor is bypassed adequately for the entire audio frequency spectrum it is desired to reproduce. If resistor 2| were omitted the bypass condensers 30 and 3| would have to be very large to prevent this degeneration. They are present to reduce such degeneration as does exist, but if as large a portion of the bias, as is consistent with distortion reduction, be made common to both tubes less degeneration occurs. In practice, the adjustment of the relative magnitude of resistors 29 and 4B and of 2| may be made as follows. Initially, resistors 20 and 40 are omitted, and resistor 2| set at the minimum value allowed by the safe plate dissipation of the'tubes in use. The distortion at some output at, or near, maximum rating is then noted. The value of resistor 2|, is then, temporarily reduced until the distortion becomes a minimum at the same output as for the original value of 2|. If, now, resistor 2| be replaced by resistors 26 and ll], each of twice the value found for resistor 2| under conditions of minimum distortion, it will be found that the plate current of each tube is less than when the bias was obtained from the common resistor. This is due to the changed rectification conditions as explained previously. If

this plate current is higher than the safe value for permissible plate dissipation, resistors 20 and '4!) must be increased to a value which results in plate dissipation within the safe limit. If, however, as frequently occurs in practice, the value 'of plate current found, when resistors 25 and 40 are adjusted for minimum distortion, is less than the allowable maximum, part of the resistance is placed in resistor 2| and part in resistors Ell and 50. In general, it is desirable to include as much of the resistance in 2| as is consistent with distortion and safe plate dissipation in order to reduce degeneration. In some cases experimentally investigated, approximately half the bias voltage was due to resistor 2|, and half due to resistors 20. and 40.

By prop-er proportioning of resistors 2 I, 22! and 40 the desired change in bias with audio signal can be secured to eliminate distortion without exceeding the safe dissipation of the tubes. For example, in a triode connected type 6196 pushpull amplifier with 350 plate volts, the minimum plate load on the driver tube should be 10,000 ohms, and if a single resistor be used for bias its minimum value is 730 ohms. If this resistor is divided, portion 2| may be 350 ohms, and 2|] and 40 be 600 ohms each, giving less distortion than if a single 650 ohm resistor common to both cathodes were used, and at the same time giving less plate dissipation. When tube 5 has peak positive voltage on its grid, the grid current will be of the order of 2 ma; Whereas the peak plate current will be of the order of 1'70 ma; so that the contribution of grid current to the bias is small.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claim.

What I claim is:

In combination with a source of audio frequency voltage, an audio transformer having a relatively low impedance, said source being connected across the transformer primary winding, 2, pair of push-pull connected tubes having their control grids connected to opposite terminals of the transformer secondary winding, a common output circuit for the plates of said tubes, said tubes being operated under such conditions that grid current flows in each tube during alternate halves of the audio cycle, biasing networks for said tubes comprising a resistive element in the space current path of one tube and connected between its cathode and the midpoint of said secondary, a second resistive element in the space current path of the second tube and connected between its cathode and said mid-point, both of said elements having a common resistive section, the magnitudes of the bias voltages developed by said networks being such that the safe operating plate dissipation of the tubes is not exceeded, and the value of the resistance of said common section being such that the distortion of the output Wave form due to grid current in one tube causing plate current cut-off in the other tube is a minimum.

DUDLEY E. FOSTER. 

